Field of the Invention
The present description relates to ultrasonic welding apparatuses for welding web materials that move continuously along a main direction.
The present description has been developed with particular attention to the sector of sanitary products. The description is, however, capable of being applied to many other sectors such as, for example, the automatic packaging industry.
Description of Prior Art
Ultrasonic welding has been employed for some time in various sectors of the art, in practice for use in all situations in which applying ultrasonic vibrational energy is possible to obtain the localized fusion of the materials.
The ultrasonic devices for carrying out the welding of two or more strips of superimposed material apply a forging-type process. That is, the welding is implemented by repeatedly striking the layers of material interposed between a vibrating device of the ultrasound system (or sonotrode) and the protuberances of a corresponding contrast (or anvil) element.
Due to the repeated blows inflicted by the vibrating element against the anvil element, the material interposed between them heats up and melts locally, exactly at the protuberances of the contrast element.
Over the years, an increasing interest has developed in the possibility of carrying out ultrasonic welding operations in dynamic production processes, typically on a web material that advances at a fairly high speed.
It is obvious that to obtain an acceptable weld in these cases it is necessary to provide a minimal amount of ultrasonic energy which, however, can vary greatly depending on the type of materials which have to be welded, the number of sheets of which they are composed and the shape and the number of protuberances of the anvil element.
In fact, relative to the latter point, it is known that technologies for forming continuous lines of ultrasonic welds, which extend parallelly to the advancing direction of the web material are consolidated and give excellent results.
However, this is not true when it comes to forming welds that extend transversely to the advancing direction of the material and that are spaced apart from each other, known as intermittent welds.
In the latter case, the ultrasonic welding technology encounters a series of difficulties due to the fact that the intermittent welds highlight the sensitivity of ultrasonic systems with respect to some characteristic parameters of the welding process. One of these parameters is that regarding the relative position (or gap) of the sonotrode relative to the outer surface of the anvil element, between which the web material subjected to welding advances.
Another extremely important factor is the permanence time of the material between the components of the welding device, during which the vibrating element or sonotrode provides the ultrasonic energy required to produce a quality weld. However, it is evident that the aforesaid permanence time decreases with increasing feeding speeds of the material.
To attempt to eliminate, or at least reduce, the aforesaid criticalities, much energy has been expended, as demonstrated by numerous documents such as, for example, EP-A-0 702 622, U.S. Pat. No. 7,383,865, EP-A-0 920 977, in which solutions are described which maintain the gap between the sonotrode and the anvil element constant during the welding operations, or which increase the permanence time of the web material in the ultrasonic device during the welding step, by instantaneously varying the advancing speed of the web material.
The technical solutions found, although having given positive results from a qualitative point of view of the welds, have ample room for improvement and numerous drawbacks linked, in particular, to the complexity of the mechanical devices and the control of the proposed solutions, both regarding the relative position (or gap) of the sonotrode relative to the anvil element and regarding the instantaneous variation devices of the speed of the web material to increase the permanence time.
These latter devices, among other things, can also present the drawback of inducing unwanted tension variations in the web material to be welded, which result in an uncontrolled variability of the length of the finished product.